1. Field of the Invention
The present invention relates to an image capturing unit, a color measuring device, an image forming apparatus, a color measuring system, and a color measurement method.
2. Description of the Related Art
As improvement in image quality has been made, image forming apparatuses, such as color inkjet image forming apparatuses and color electrophotographic image forming apparatuses, have come to be used for offset printing of advertising media, brochures, and the like that require high quality images in a relatively small number of copies.
In the case of the offset printing requiring a high image quality, there are cases in which colors of a printed product required by a client differ from colors of printed-out results actually printed out by an image forming apparatus.
While a client normally checks colors of a printed product on a display and then places an order for printing, each image forming apparatus has color reproduction characteristics specific to the model thereof. This may lead to printed results having different colors from those checked on the display.
To address this, techniques have been used that reproduce colors by using a color space, such as an L*a*b* color space or an XYZ color space, that does not depend on devices such as a display and an image forming apparatus.
An image forming apparatus controls amounts of color materials and the like to output specified colors. For example, an inkjet image forming apparatus arithmetically controls ink discharge amounts, print patterns, and the like to control the amounts of ink discharged from ink heads so as to control output colors. An electrophotographic image forming apparatus controls amounts of toner on photosensitive elements, light intensity of laser beams thereon, and the like so as to control output colors.
Amounts of color materials such as ink discharge amounts of an inkjet image forming apparatus, however, fluctuate due to factors such as states of nozzles of the heads, fluctuation in viscosity of ink, and fluctuation of discharge driving elements (such as piezoelectric elements), thus causing fluctuation in color reproducibility. Ink discharge amounts of an inkjet image forming apparatus change with time in one image forming apparatus and differ between image forming apparatuses, thereby generating fluctuation in color reproduction of images with time and between image forming apparatuses.
To address this, an image forming apparatus has conventionally performed color adjustment processing to reduce fluctuation in output due to device-specific characteristics so as to enhance the reproducibility of input in output. In this color adjustment processing, for example, the image forming apparatus first actually outputs an image of a color patch having a reference color (reference color patch image), and a color measuring device measures the color of the reference color patch image. Then, based on a difference between the color measurement value of the reference color patch image measured by the color measuring device and a color specification value in a standard color space of a corresponding standard color, a color conversion parameter is generated and set to the image forming apparatus. Thereafter, based on the thus set color conversion parameter, the image forming apparatus applies, when outputting an image corresponding to input image data, color conversion to the input image data. Based on this color-converted image data, the image forming apparatus then records and outputs the image. This reduces fluctuation in output due to device-specific characteristics so as to output an image with high color reproducibility.
In this conventional color adjustment processing, a spectrophotometric colorimeter is widely used as the color measuring device which measures the color of a reference color patch image. A spectrophotometric colorimeter can perform highly accurate color measurement because of capability of obtaining a spectral reflectance factor for each wavelength. A spectrophotometric colorimeter is, however, an expensive device, and thus, it is desired to make it possible to perform highly accurate color measurement using a lower cost device.
Hence, there has been conventionally disclosed a color measuring device which includes a reference color measuring unit that measures, in advance, a color of a reference color patch so as to obtain a color reference value as RGB data; a color image input unit that captures an image of an image capture object including the reference color patch and a target for color measurement simultaneously or separately so as to obtain RGB data; an image extracting unit that extracts the RGB data of the reference color patch and the RGB data of the target for color measurement from the RGB data obtained by the color image input unit; and an arithmetic unit that obtains a difference between the RGB data of the reference color patch obtained by the image extracting unit and the RGB data of the reference color patch obtained by the reference color measuring unit and then uses the difference to correct at least the RGB data of the target for color measurement for color (refer to Japanese Patent No. 3129502). This conventional technique discloses placing the reference color patch to be compared with the image capture object near the image capture object that is a target for color measurement; simultaneously capturing the images of the image capture object and of the reference color patch using a color video camera serving as the color image input unit; correcting the RGB data of the image capture object by using the RGB data of the reference color patch obtained by the image capturing; and converting the RGB data of the image capture object into the color specification value in the standard color space.
This technique described in Japanese Patent No. 3129502 has, however, a difficulty in maintaining a constant positional relation among the image capture object, the reference color patch, and the color video camera. Thus, image capturing conditions can change each time image capturing is performed, resulting in impossibility of stable image capturing.
In view of the above description, there is a need to provide an image capturing unit, a color measuring device, an image forming apparatus, a color measuring system, and a color measurement method that enable stable image capturing.